You will need 2 people. One goes under the dash and looks for a pin connection that is on the climate controls or a wire connection board that is missing a wire. There will be more than one. Take a 12 volt light bulb and solder a wire in the centre and another on the outer casing of bulb. Connect one of these to the vehicle ground. (Any metal piece of the chassis will usually do but the best is the negative post of the battery) Take the other wire and touch each of the pins that are missing a wire. As you touch each wire, get your buddy to switch the heater fan off and on. When you touch the right one the light bulb will light up. Hook the loose wire to that pin.